If you knock them out, the mice die of massive systemic autoimmune reactions.

Utilizing these cells to prevent GVHD has been a goal of scientists and doctors for some time now. The challenge is that they, like all other T-cells, have an antigen-specific TCR. Naturally occuring Tregs need to see cognate (in this case self) antigen to elicit suppressive effects. An imperfect graft match does not provide these antigens, and thus protection from the host's Tregs is not elicited. Instead, most of these antigens are seen as forgin and elicit the reaction of conventional effector cells.